ENIAC was formally dedicated at the University of Pennsylvania on February 15, 1946 and was heralded as a "Giant Brain" by the press.[8] It had a speed on the order of one thousand (103) times faster than that of electro-mechanical machines; this computational power, coupled with general-purpose programmability, excited scientists and industrialists alike. This combination of speed and programmability allowed for thousands more calculations for problems, as ENIAC calculated a trajectory that took a human 20 hours in 30 seconds (a 2400x increase in speed).[9]

ENIAC was designed by John Mauchly and J. Presper Eckert of the University of Pennsylvania, U.S.[12] The team of design engineers assisting the development included Robert F. Shaw (function tables), Jeffrey Chuan Chu (divider/square-rooter), Thomas Kite Sharpless (master programmer), Frank Mural (master programmer), Arthur Burks (multiplier), Harry Huskey (reader/printer) and Jack Davis (accumulators).[13] In 1946, the researchers resigned from the University of Pennsylvania and formed the Eckert-Mauchly Computer Corporation.

ENIAC was a modular computer, composed of individual panels to perform different functions. Twenty of these modules were accumulators which could not only add and subtract, but hold a ten-digit decimal number in memory. Numbers were passed between these units across several general-purpose buses (or trays, as they were called). In order to achieve its high speed, the panels had to send and receive numbers, compute, save the answer and trigger the next operation, all without any moving parts. Key to its versatility was the ability to branch; it could trigger different operations, depending on the sign of a computed result.

By the end of its operation in 1955, ENIAC contained 17,468 vacuum tubes, 7200 crystal diodes, 1500 relays, 70,000 resistors, 10,000 capacitors and approximately 5,000,000 hand-soldered joints. It weighed more than 30 short tons (27 t), was roughly 2.4m × 0.9m × 30m (8 × 3 × 100 feet) in size, occupied 72m2 (1800 ft2) and consumed 150 kW of electricity.[14][15] This power requirement led to the rumor that whenever the computer was switched on, lights in Philadelphia dimmed.[16] Input was possible from an IBM card reader and an IBM card punch was used for output. These cards could be used to produce printed output offline using an IBM accounting machine, such as the IBM 405. While ENIAC had no system to store memory in its inception, these punch cards could be used for external memory storage.[17] In 1953, a 100-word magnetic-core memory built by the Burroughs Corporation was added to ENIAC.[18]

ENIAC used ten-position ring counters to store digits; each digit required 36 vacuum tubes, 10 of which were the dual triodes making up the flip-flops of the ring counter. Arithmetic was performed by "counting" pulses with the ring counters and generating carry pulses if the counter "wrapped around," the idea being to electronically emulate the operation of the digit wheels of a mechanical adding machine.

ENIAC had 20 ten-digit signed accumulators, which used ten's complement representation and could perform 5000 simple addition or subtraction operations between any of them and a source (e.g., another accumulator or a constant transmitter) every second. It was possible to connect several accumulators to run simultaneously, so the peak speed of operation was potentially much higher, due to parallel operation.

It was possible to wire the carry of one accumulator into another accumulator to perform double precision arithmetic, but the accumulator carry circuit timing prevented the wiring of 3+ for even higher precision. ENIAC used 4 of the accumulators (controlled by a special multiplier unit) to perform up to 385 multiplication operations/second; 5 of the accumulators were controlled by a special divider/square-rooter unit to perform up to 40 division operations/second or 3 square root operations/second.

The other 9 units in ENIAC were the Initiating Unit (started and stopped the machine), the Cycling Unit (used for synchronizing the other units), the Master Programmer (controlled "loop" sequencing), the Reader (controlled an IBM punch-card reader), the Printer (controlled an IBM card punch), the Constant Transmitter and 3 function tables.[clarification needed]

Cpl. Irwin Goldstein (foreground) sets the switches on one of ENIAC's function tables at the Moore School of Electrical Engineering. (U.S. Army photo) [19]

The references by Rojas and Hashagen (or Wilkes)[12] give more details about the times for operations, which differ somewhat from those stated above.

The basic machine cycle was 200 microseconds (20 cycles of the 100 kHz clock in the cycling unit), or 5,000 cycles per second for operations on the 10-digit numbers. In one of these cycles, ENIAC could write a number to a register, read a number from a register, or add/subtract two numbers.

A multiplication of a 10-digit number by a d-digit number (for d up to 10) took d+4 cycles, so a 10- by 10-digit multiplication took 14 cycles, or 2800 microseconds—a rate of 357 per second. If one of the numbers had fewer than 10 digits, the operation was faster.

Division and square roots took 13(d+1) cycles, where d is the number of digits in the result (quotient or square root). So a division or square root took up to 143 cycles, or 28,600 microseconds—a rate of 35 per second. (Wilkes 1956:20[12] states that a division with a 10 digit quotient required 6 milliseconds.) If the result had fewer than ten digits, it was obtained faster.

Several tubes burned out almost every day, leaving it nonfunctional about half the time. Special high-reliability tubes were not available until 1948. Most of these failures, however, occurred during the warm-up and cool-down periods, when the tube heaters and cathodes were under the most thermal stress. Engineers reduced ENIAC's tube failures to the more acceptable rate of one tube every two days. According to a 1989 interview with Eckert, "We had a tube fail about every two days and we could locate the problem within 15 minutes."[21] In 1954, the longest continuous period of operation without a failure was 116 hours—close to five days.

ENIAC could be programmed to perform complex sequences of operations, including loops, branches, and subroutines. However, instead of the stored program computers that exist today, ENIAC was just a large collection of arithmetic machines,[22] which had programs hard coded into the machines with function tables that each contained 1200 ten way switches.[23] The task of taking a problem and mapping it onto the machine was complex, and usually took weeks. Due to the complexity of mapping programs onto the machine, programs were only changed after huge numbers of tests of the current program.[24] After the program was figured out on paper, the process of getting the program into ENIAC by manipulating its switches and cables could take days. This was followed by a period of verification and debugging, aided by the ability to execute the program step by step. A programming tutorial for the modulo function using an ENIAC simulator gives an impression of what a program on the ENIAC looked like.[25]

Kay McNulty,Betty Jennings, Betty Snyder, Marlyn Meltzer, Fran Bilas, and Ruth Lichterman were the first programmers of the ENIAC, though their work was not widely recognized for over 50 years. At the time, the hardware was seen as the primary innovation and the complexity of programming the machine was undervalued, perhaps because the first programmers were all women. Historians had at first mistaken them for "Refrigerator Ladies" (models posing in front of the machine), and they are sometimes referred to as the "ENIAC Girls" since referring to adult women as girls was common in the 1940s[29] Most of the women did not receive recognition for their work on the ENIAC in their lifetimes.[30] 50 years after the invention of the ENIAC most of the female programmers were not invited to the 50th anniversary event.[29]

These early programmers were drawn from a group of about two hundred female "computers" who studied at the Moore School of Electrical Engineering at the University of Pennsylvania. One of the few technical job categories available to women was computing the results of mathematical formulas for science and engineering, usually with a mechanical calculator.[31]Betty Holberton (nee Snyder) continued on to invent the first sorting algorithm and help design the first commercial computers, the UNIVAC and the BINAC, alongside Jean Jennings.

Herman Goldstine selected the programmers, then called "operators", from the human "computers" who had been calculating ballistics tables with desk calculators and a differential analyzer prior to and during the development of ENIAC.[30] Under Herman and Adele Goldstine's direction, the programmers studied ENIAC's blueprints and physical structure to determine how to manipulate its switches and cables, rather than learning a programming language. Though contemporaries considered programming a clerical task and did not publicly recognize the programmers' impact on the successful operation and announcement of ENIAC,[30] McNulty, Jennings, Snyder, Wescoff, Bilas, and Lichterman have since been recognized for their contributions to computing.[32][33][34]

Following the initial six programmers, an expanded team of a hundred scientists was recruited to continue work on the ENIAC. Among these were several women, including Gloria Ruth Gordon.[35]

Although the Ballistic Research Laboratory was the sponsor of ENIAC, one year into this three-year project John von Neumann, a mathematician working on the hydrogen bomb at Los Alamos National Laboratory, became aware of this computer.[36]Los Alamos subsequently became so involved with ENIAC that the first test problem ran consisted of computations for the hydrogen bomb, not artillery tables.[6] The input/output for this test was one million cards.[37]

Related to ENIAC's role in the hydrogen bomb was its role in the Monte Carlo method becoming popular. Scientists involved in the original nuclear bomb development used massive groups of people doing huge numbers of calculations ("computers" in the terminology of the time) to investigate the distance that neutrons would likely travel through various materials. John von Neumann and Stanislaw Ulam realized the speed of ENIAC would allow these calculations to be done much more quickly. The success of this project showed the value of Monte Carlo methods in science.[38]

The completed machine was announced to the public the evening of February 14, 1946[39] and formally dedicated the next day[40] at the University of Pennsylvania. The original contract amount was $61,700; the final cost was almost $500,000 (approximately $6,100,000 today). It was formally accepted by the U.S. Army Ordnance Corps in July 1946. ENIAC was shut down on November 9, 1946 for a refurbishment and a memory upgrade, and was transferred to Aberdeen Proving Ground, Maryland in 1947. There, on July 29, 1947, it was turned on and was in continuous operation until 11:45 p.m. on October 2, 1955.[2]

A few months after ENIAC's unveiling, in the summer of 1946, as part of "an extraordinary effort to jump-start research in the field",[41]the Pentagon invited "the top people in electronics and mathematics from the United States and Great Britain"[41] to a series of forty-eight lectures altogether called The Theory and Techniques for Design of Digital Computers more often named the Moore School Lectures.[41] Half of these lectures were given by the inventors of ENIAC.[42]

ENIAC was a one-of-a-kind design and was never repeated. The freeze on design in 1943 meant that the computer design would lack some innovations that soon became well-developed, notably the ability to store a program. Eckert and Mauchly started work on a new design, to be later called the EDVAC, which would be both simpler and more powerful. In particular, in 1944 Eckert wrote his description of a memory unit (the mercury delay line) which would hold both the data and the program. John von Neumann, who was consulting for the Moore School on the EDVAC, sat in on the Moore School meetings at which the stored program concept was elaborated. Von Neumann wrote up an incomplete set of notes (First Draft of a Report on the EDVAC) which were intended to be used as an internal memorandum describing, elaborating, and couching in formal logical language the ideas developed in the meetings. ENIAC administrator and security officer Herman Goldstine distributed copies of this First Draft to a number of government and educational institutions, spurring widespread interest in the construction of a new generation of electronic computing machines, including Electronic Delay Storage Automatic Calculator (EDSAC) at Cambridge University, England and SEAC at the U.S. Bureau of Standards.

A number of improvements were made to ENIAC after 1948, including a primitive read-only stored programming mechanism[43] using the Function Tables as program ROM, an idea included in the ENIAC patent and proposed independently by Dr. Richard Clippinger of BRL. Clippinger consulted with von Neumann on what instruction set to implement. Clippinger had thought of a 3-address architecture while von Neumann proposed a 1-address architecture because it was simpler to implement. Three digits of one accumulator (6) were used as the program counter, another accumulator (15) was used as the main accumulator, a third accumulator (8) was used as the address pointer for reading data from the function tables, and most of the other accumulators (1–5, 7, 9–14, 17–19) were used for data memory.

The programming of the stored program for ENIAC was done by Betty Jennings, Clippinger and Adele Goldstine. It was first demonstrated as a stored-program computer on September 16, 1948, running a program by Adele Goldstine for John von Neumann. This modification reduced the speed of ENIAC by a factor of six and eliminated the ability of parallel computation, but as it also reduced the reprogramming time to hours instead of days, it was considered well worth the loss of performance. Also analysis had shown that due to differences between the electronic speed of computation and the electromechanical speed of input/output, almost any real-world problem was completely I/O bound, even without making use of the original machine's parallelism. Most computations would still be I/O bound, even after the speed reduction imposed by this modification.

Early in 1952, a high-speed shifter was added, which improved the speed for shifting by a factor of five. In July 1953, a 100-word expansion core memory was added to the system, using binary coded decimal, excess-3 number representation. To support this expansion memory, ENIAC was equipped with a new Function Table selector, a memory address selector, pulse-shaping circuits, and three new orders were added to the programming mechanism.

A function table from ENIAC on display at Aberdeen Proving Ground museum.

Mechanical and electrical computing machines have been around since the 19th century, but the 1930s and 1940s are considered the beginning of the modern computer era.

ENIAC was, like the Z3 and Harvard Mark I, able to run an arbitrary sequence of mathematical operations, but did not read them from a tape. Like the Colossus, it was programmed by plugboard and switches. ENIAC combined full, Turing complete programmability with electronic speed. The Atanasoff–Berry Computer (ABC), ENIAC, and Colossus all used thermionic valves (vacuum tubes). ENIAC's registers performed decimal arithmetic, rather than binary arithmetic like the Z3, the ABC and Colossus.

Like the Colossus, ENIAC required rewiring to reprogram until September 1948. Three months earlier, in June 1948, the Manchester Small-Scale Experimental Machine (SSEM) ran its first program and earned the distinction of first stored-program computer. Though the idea of a stored-program computer with combined memory for program and data was conceived during the development of ENIAC, it was not initially implemented in ENIAC because World War II priorities required the machine to be completed quickly, and ENIAC's 20 storage locations would be too small to hold data and programs.

The Z3 and Colossus were developed independently of each other, and of the ABC and ENIAC during World War II. The Z3 was destroyed by the Allied bombing raids of Berlin in 1943. The ten Colossus machines were part of the UK's war effort. Their existence only became generally known in the 1970s, though knowledge of their capabilities remained among their UK staff and invited Americans. All but two of the machines that remained in use in GCHQ until the 1960s, were dismantled in 1945.[44] Work on the ABC at Iowa State University was stopped in 1942 after John Atanasoff was called to Washington, D.C., to do physics research for the U.S. Navy, and it was subsequently dismantled.[45]

ENIAC, by contrast, was put through its paces for the press in 1946, "and captured the world's imagination". Older histories of computing may therefore not be comprehensive in their coverage and analysis of this period. As the Colossus was used to decrypt Russian messages up until the 1960's, it was kept confidential for almost 40 years, only becoming public knowledge in the late 1970's.[citation needed]

For a variety of reasons (including Mauchly's June 1941 examination of the Atanasoff–Berry Computer, prototyped in 1939 by John Atanasoff and Clifford Berry), U.S. Patent 3,120,606 for ENIAC, applied for in 1947 and granted in 1964, was voided by the 1973 decision of the landmark federal court case Honeywell v. Sperry Rand, putting the invention of the electronic digital computer in the public domain and providing legal recognition to Atanasoff as the inventor of the first electronic digital computer.

In 1996, in honor of the ENIAC's 50th anniversary, The University of Pennsylvania sponsored a project named, "ENIAC-on-a-Chip", where a very small silicon computer chip measuring 7.44 mm by 5.29 mm was built with the same functionality as ENIAC. Although this 20 MHz chip was many times faster than ENIAC, it had but a fraction of the speed of its contemporary microprocessors in the late 1990s.[48][49][50]

^Scott McCartney p.103 (1999): "ENIAC correctly showed that Teller's scheme would not work, but the results led Teller and Ulam to come up with another design together."

^Brain used in the press as a metaphor became common during the war years. Looking, for example, at Life magazine: August 16, 1937 p.45 Overseas Air Lines Rely on Magic Brain (RCA Radiocompass). March 9, 1942 p.55 the Magic Brain—is a development of RCA engineers (RCA Victrola). December 14, 1942 p.8 Blanket with a Brain does the rest! (GE Automatic Blanket). November 8, 1943 p.8 Mechanical brain sights gun (How to boss a BOFORS!)

^Honeywell, Inc. v. Sperry Rand Corp., 180 U.S.P.Q. (BNA) 673, p. 20, finding 1.1.3 (U.S. District Court for the District of Minnesota, Fourth Division 1973) (“The ENIAC machine which embodied 'the invention' claimed by the ENIAC patent was in public use and non-experimental use for the following purposes, and at times prior to the critical date: ... Formal dedication use February 15, 1946 ...”).

^Scott McCartney p.140 (1999): Eckert gave 11 lectures, Mauchly gave 6, Goldstine gave 6, von Neumann, who was to give 1, didn't showup. the other 24 were spread among various invited academics and military officials.

Mauchly, John, The ENIAC (in Metropolis, Nicholas, J. Howlett, Gian-Carlo Rota, 1980, A History of Computing in the Twentieth Century, Academic Press, New York, ISBN 0-12-491650-3, pp. 541–550, "Original versions of these papers were presented at the International Research Conference on the History of Computing, held at the Los Alamos Scientific Laboratory, 10–15 June 1976.")

Interview with Eckert Transcript of a video interview with Eckert by David Allison for the National Museum of American History, Smithsonian Institution on February 2, 1988. An in-depth, technical discussion on ENIAC, including the thought process behind the design.

Oral history interview with Irven A. Travis, Charles Babbage Institute, University of Minnesota. Travis describes the ENIAC project at the University of Pennsylvania (1941–46), the technical and leadership abilities of chief engineer Eckert, the working relations between John Mauchly and Eckert, the disputes over patent rights, and their resignation from the university. Oral history interview by Nancy B. Stern, 21 October 1977.

Oral history interview with S. Reid Warren, Charles Babbage Institute, University of Minnesota. Warren served as supervisor of the EDVAC project; central to his discussion are J. Presper Eckert and John Mauchly and their disagreements with administrators over patent rights; discusses John von Neumann's 1945 draft report on the EDVAC, and its lack of proper acknowledgment of all the EDVAC contributors.